Sintered Aluminum Powder (SAP)

Abstract:

Sintered aluminum powder alloys have properties quite different from those of material fabricated by conventional techniques. The oxide that forms immediately on the surface of aluminum is not reduced back to metal during sintering and the resulting powder product contains a substantial amount of oxide. This oxide prevents grain growth and movement of dislocations at the boundaries or through them and produces high strength, high creep resistance and insensitivity to high-temperature exposure.

Sintered aluminum powder alloys have properties quite different from those of
material fabricated by conventional techniques. The oxide that forms immediately
on the surface of aluminum is not reduced back to metal during sintering and the
resulting powder product contains a substantial amount of oxide. This oxide prevents
grain growth and movement of dislocations at the boundaries or through them and
produces high strength, high creep resistance and insensitivity to high-temperature
exposure.

The amount of oxide varies:

Flake powder may contain as much as 20% Al2O3;

Atomized powders a few percent;

Electrolytic powders have intermediate contents;

Ball milled powders are denser and consist of small particles welded together
so that the oxide is present not only at the surface but also within the particles.

The material properties depend on the amount of naturally formed oxide. Heating
powder to increase the thickness of the oxide film or addition of
Al2O3 powder, however, does not increase
strength and only reduces ductility.

Reportedly, SAP-type products can also be made by dispersing Al2O3 into molten
aluminum with ultrasonic vibration or by blowing reducible oxides of high melting
point metals into an aluminum melt. Powders milled wet with hydrocarbons contain
Al4C3 in addition to oxide.

The natural oxide formed on the powder is some 100 x 10-10m thick, is
amorphous and contains absorbed water. Upon heating at high temperature the oxide
crystallizes first to η(Al2O3), then to χ(Al2O3) or γ`(Al2O3) and
finally to the stable γ(Al2O3).

The absorbed water reacts with the metal to form additional oxide and release
hydrogen. This hydrogen may produce porosity at the grain boundaries and cracking
or blistering. The higher the oxide content, the more hydrogen is released and
the more pronounced the embrittling and defect formation, especially with cyclic
heating and cooling. Vacuum treatment or high-temperature sintering before complete
compacting reduces hydrogen content and eliminates most if not all cracking. Small
additions of aluminum fluoride also reduce the effect of hydrogen.

The oxide is present as finely dispersed particles, which interact with vacancies
and dislocations and prevent their easy movement. The oxide effect persists even
above the melting point of aluminum.

The structure of SAP materials depends on the origin of the
powder, but mainly on fabrication technique and oxide content. The normal
iron and silicon compounds present in aluminum fragment in powdering and
become dispersed in the metal. They act as points of weakness and starting
points for fracture, but the improvement of properties obtained by the use
of Al 99.99% is limited.

The density of the powders themselves is of the order of 900-1000 kg/m3,
but when compacted to 95% or better, the density is 2710-2720 kg/m3,
slightly higher than that of aluminum. Substantial additions of heavy metals raise
the density further.

The thermal expansion coefficient is lower than for pure aluminum
and decreases almost linearly with increasing oxide content, to reach values of
the order of 20x10-61/K for the 300-600 K range at 15% oxide. Thermal
conductivity decreases by approximately 1% for every 1% of oxide present, but is
higher in the direction of extrusion. Repeated pressing with intermediate vacuum
annealing gives the maximum conductivity. The Lorentz constant is
23 x 10-6W/Ω/K2.

Electric resistivity strongly depends on structure: if the oxide film is unbroken,
as in the case after pressing and sintering only, resistivities as high as
1 Ωm have been measured; if, on the other hand, very-high-temperature sintering
(>800K) or extrusion is used to break up the film, the resistivity drops by
some 5-6 orders of magnitude to values ranging from 2.9 x 10-8 Ωm
for 1-2% oxide to 4-4.5 x 10-8 Ωm at 15% oxide. The increase in
resistivity with temperature parallels that of aluminum up to ≈900K, where it
becomes steeper. Quenching from high temperature increases the resistivity because
the impurities (silicon, iron, manganese, etc.) are retained in solution.
Resistivities of alloys are higher, especially if made from atomized powders.

Grain strength is proportional to the area of contact of the
particles; strength after sintering is directly and ductility inversely proportional
to oxide content. Grain size of the sintered product has little or no effect, but
coarse grain produces an increase in strength and ductility at high temperature.
The mosaic block size in the matrix, together with distance between oxide particles,
controls the properties. Typical properties at various temperatures are shown in the
Table 1.

A tendency for intercrystalline fracture is reported due to separation of metal
from oxide. High-temperature sintering tends to reduce strength and increase
ductility. Up to 20-40% cold working increases strength but then further work
breaks up the oxide network and tends to facilitate coalescence of the oxide films.
Consequently, strength and creep resistance decrease. Material rolled to foil has
properties close to those of pure aluminum.

Table 1: Mechanical properties of SAP as a function of temperature

Property

%oxide

Temperature, K

170

300

500

600

700

800

900

UTSMPa

1

-

80-140

-

60-80

40-70

-

-

3

-

150-220

-

-

60-80

-

-

7

350

230-280

120-150

100-140

70-100

50-70

-

12

400-500

320-380

180-220

150-190

100-140

60-90

-

15

-

400-500

200-250

150-200

100-150

70-100

40

YSMPa

1

-

40-70

-

40-60

-

-

-

3

-

100-140

-

-

-

-

-

7

-

120-160

100-140

80-120

60-90

40-80

-

12

250-300

180-240

140-180

110-150

80-120

60-90

-

15

-

200-260

170-220

140-200

100-150

80-100

30

%A

1

-

25-30

-

-

27-34

-

-

3

-

18-24

-

-

14-18

-

-

7

-

14-18

14-18

12-15

4-8

2-4

-

12

-

8-12

4-9

3-6

3-6

2-5

-

15

-

5-9

5-8

5-8

3-5

2-4

2

HV

1

-

300

-

200-250

-

-

-

3

-

500

-

-

-

-

-

7

-

550-700

-

-

-

-

-

12

-

900-1000

-

-

-

-

-

15

-

1000-1100

600

500

400

300

100

Fatigue strength is of the order of 60-70 MPa at 107
cycles and the decrease with temperature parallels that of the strength. Fatigue
resistance is increased by some 10-20% by high (12-14 ppm) hydrogen in solution
but decreased sharply by notches and slow strain rates. It is higher in vacuum.

Creep resistance is extremely high and exceeds that of all aluminum alloys.
Activation energy for creep of 6.5 eV has been reported. Impact strength rises
with increasing temperature up to 800-850 K and then declines; shear strength
behaves similarly.

Another important characteristic of sintered aluminum powders is their
insensitivity to high temperature: exposure for several years at temperatures
up to 800 K produces practically no change in structure or properties, especially
in the higher-oxide-content alloys.

The modulus of elasticity increases with oxide content to reach values of the
order of 77-80 GPa at 15-16% Al2O3, declining with temperature
as does the strength. The damping capacity of SAP is some 20 times higher
than that of aluminum. Abrasion resistance does not differ substantially
from that of pure aluminum. Neutron or ion irradiation hardens the material
but not enough to prevent use in nuclear reactors.

The potential of sintered products is practically the same as that of commercial
aluminum; the oxide has little effect on the pitting potential. Aluminum 99.99%
is anodic to SAP but aluminum 99.3% is not. Corrosion resistance is slightly worse
than that of the corresponding wrought product. It is good for products containing
only oxide and manganese, chromium and magnesium; somewhat lower with additions of
silicon, iron, nickel, etc.; poor if copper and tin are added. Oxide distribution
and content affect somewhat the corrosion resistance of SAP to water at elevated
temperatures. However, products with iron, nickel and tungsten show the same
(or slightly better) resistance of the corresponding alloys fabricated by
conventional methods. Cladding with aluminum, preferably 99.99% pure or
aluminum-magnesium alloys improves corrosion resistance to seawater.

Alloys containing large amounts (up to 20%) of chromium, nickel, cobalt, iron,
manganese, titanium and titanium carbide have moduli of elasticity of up to
100-120 GPa and high creep resistance, but if prepared by mixing aluminum powder,
and metal, they are brittle and the compounds that form in sintering tend to
fragment when deformed. On the other hand, if they are prepared from alloy
powders, their properties and corrosion resistance are better, especially if
the rapid cooling or atomizing is exploited to supersaturate the aluminum, and
no segregation is present.

Additions of SiO2, SiC, B4C
and AlPO4 embrittle the material without a corresponding
increase in strength. Chromium, iron, tungsten, etc. oxides or carbonates, which
react with aluminum to increase the amount of A12O3
and liberate the corresponding metal, on the other hand, increase strength. Alloys
containing up to 50% Si have low density and low expansion coefficients. Mixtures
containing boron or boron carbide can be used to extrude moderator rods for atomic
reactors.

High-diffusivity, elements, whose compounds tend to coalesce on
high-temperature exposure, lose strength rapidly. Thus, dural, aluminum-copper
and aluminum-zinc-magnesium alloys made by powder metallurgy have a strength at
room temperature some 30-50% higher than the corresponding aluminum, but after
6 months exposure at 500-550 K have lost their extra strength at room temperature
and at higher temperatures are weaker and less creep resistant. Age hardening of
SAP alloys is not different from that of normal alloys.

Texture in extrusion tends to be of aluminum, but less pronounced, especially with
fine size powders. In sheet a variety of textures has been reported:
(111) [112]; (110) [447]; (315) [513]. The use of backpressure on extrusion
produces materials with higher high-temperature strength. High-speed
extrusion (2-4 m/sec) produces a coarser structure but no substantial
difference in properties.

Both deformation and creep mechanisms change with temperature. Slip is on the
(111) and (211) planes, with [01-1] and [206] directions.

Recovery of SAP is very similar to that of pure aluminum, but the mosaic
structure is smaller, because dislocations tend to be pinned at oxide particles.
Low-oxide materials (<3% Al2O3) recrystallise easily,
but the recrystallisation temperature rises steeply with oxide content, so that
above 5-7% Al2O3 recrystallisation, especially of extrusions,
is very rare. Supersaturation with Fe, Mn and
Ni further increases the recrystallisation temperature. Activation
energy for grain growth is of the order of 2 eV. Diffusion of elements is
faster in SAP than in aluminum.

The data on compatibility of SAP and atomic fuels are somewhat contrasting: some
scientist report no reaction up to 900 K when SAP is used for canning uranium oxide
or carbide; reaction above 700 K, on the other hand, is reported by others.

The data on compatibility of SAP and atomic fuels are somewhat contrasting: some
scientist report no reaction up to 900 K when SAP is used for canning uranium oxide
or carbide; reaction above 700 K, on the other hand, is reported by others.

Extrusion, forging or hot rolling of compacted borings, fine chips or granules of
aluminum and alloys have been tried, either as a means of recovering machining
residues or as a short cut in the production of thin sheet, foil, or complex shapes,
but the properties obtained are very close to those of the conventional materials
rather than those made from powders.